Wheel constructions are known, for example from U.S. Pat. No. 5,560,686 which include a circular wheel rim which provides respective axially inner and outer flanges which each provide a seat for a tyre bead of a tyre which is fitted to a radially outer, or “tyre side” of the rim. In the example of U.S. Pat. No. 5,560,686, the wheel construction further includes a disc by means of which the wheel is securable to a hub of the tractor, the disc being connected to a radially inner side of the rim and the disc and rim being separable.
The rim has a mid-portion which may typically be central, where, at the tyre side, a central well is provided, and includes side parts which extend radially outwardly between the central well and the flanges, the side parts each including a pair of straight surfaces (where the rim is viewed in cross section) and a connecting part between the straight surfaces, the connecting part including a well side wall which includes a straight portion, and a curved region which is generally concave when considered from the tyre side of the rim, the well side wall and curved region providing together in the connecting part, a further well. The central well of the central region and the further well in each side part, are provided to afford the rim with the necessary strength to withstand the forces imposed on the wheel construction in use.
Wheel constructions which only have a single, usually central well, are designated as “W” (well) designs, whereas wheel constructions which provide a further well in each rim side part are designated “DW” (double well) designs.
Internationally accepted standards set down certain wheel design criteria. For example, the current European Tyre and Rim Technical Organisation (ETRTO) standard for Agricultural Equipments requires that, for a “DW” designated design, at any circumferential position around the rim, in cross section, the minimum angle between a plane normal to the wheel axis and a line along a straight portion of the well side wall is 15°. Also the ETRTO standard requires that at the tyre side of the rim, an external radius where the straight portion of the well side adjoins and melds with the adjacent axially outer straight surface, is a minimum of 9.5 mm for a typical tractor wheel, and the maximum internal radius of the concave curved region is 14.5 mm.
As tractor and the like designs have progressed, tyres are being produced which are capable of supporting greater loads, and to achieve this, tyre walls are being made stiffer. This presents problems in fitting such tyres, particularly, where there is a well side wall at an angle close to the standard minimum relative to a plane normal to the wheel axis, as one or both of the tyre beads may not immediately seat on the flanges during initial inflation. Rather, one or both of the beads may temporarily seat on the well side wall during initial inflation, until a high pressure is established in the tyre, which may suddenly be released as the bead or beads disengage the well side wall and move towards the flanges.
In certain circumstances, to overcome this situation, tyre pressures may be increased to above that recommended by the tyre manufacturer.
With current single “W” and double “DW” well rim designs, there is little scope for increasing the angle of the well side walls whilst retaining the required strength in the rim, due to the restricted axial length of the rim, which is referred to commonly as the “width” of the wheel.